In medium wave broadcasting, for example, a digital amplitude modulation apparatus is used. The digital amplitude modulation apparatus has a plurality of power amplifiers arranged in parallel, controls the plurality of power amplifiers to be ON/OFF in accordance with a voltage amplitude level of a signal to be modulated, to thereby change the number of the power amplifiers which respectively output amplified carrier signals, and combines the respective output signals of the power amplifiers in an ON state, to thereby generate an amplitude modulation signal, and transmits an amplitude modulation signal wave to a prescribed broadcast service area.
In the above-described digital amplitude modulation apparatus, when the power amplifier breaks down, or some sort of fault occurs, there is a possibility that signal quality of an AM (Amplitude Modulation) wave deteriorates. For this reason, at lightning strike, or at the time of load system abnormalities such as disconnecting, failure, and surge mixing, for example, it is necessary to prevent breakdown of the power amplifier at the time of degradation of an SWR (Standing Wave Ratio) due to load characteristic of an antenna.
So, the digital amplitude modulation apparatus, in order to prevent breakdown of the power amplifier due to reflected power from an antenna and so on, for example, monitors the SWR, and controls the output power thereof based on the SWR.
Conventionally, when the SWR exceeds a warning limit, a method to turn off a switch of a transmission circuit, to thereby make the output power zero, and a method to reduce transmission power are known.
A general micro-computer is used for measurement of an SWR. Since a CPU of the micro-computer has a large arithmetic delay amount, the CPU of the micro-computer can not follow rapid change of the SWR due to lightning strike etc. in real time.
In addition, a determining method using an SWR calculated from a reflection coefficient that is a ratio of a reflected voltage to an incident voltage can not discriminate change of the output power due to a phase component of the reflection coefficient, and thereby narrows a broadcast service cover area more than necessary.
In addition, a protection circuit may malfunction due to degradation of the SWR by a modulation wave resulting from load characteristic of an antenna.
An object of the invention is to provide a digital amplitude modulation apparatus and a control method of a digital amplitude modulation apparatus which can perform protection operation of a power amplifier without narrowing a cover range of a broadcast service area and without deteriorating signal quality of an AM wave.